What is a cohort study in medical research?

Cohort studies are a type of medical research used to investigate the causes of disease and to establish links between risk factors and health outcomes.

The word cohort means a group of people. These types of studies look at groups of people. They can be forward-looking (prospective) or backward-looking (retrospective).

Prospective" studies are planned in advance and carried out over a future period of time.

Retrospective cohort studies look at data that already exist and try to identify risk factors for particular conditions. Interpretations are limited because the researchers cannot go back and gather missing data.

These long-term studies are sometimes called longitudinal studies.

Fast facts on cohort studies

Cohort studies typically observe large groups of individuals, recording their exposure to certain risk factors to find clues as to the possible causes of disease.

They can be prospective studies and gather data going forward, or retrospective cohort studies, which look at data already collected.

The Nurses' Health Study is one example of a large cohort study, and it has produced many important links between lifestyle choices and health by following hundreds of thousands of women across North America.

Such research can also help identify social factors that influence health.

Finding causes

Cohort studies look at large groups of people to try to find out what might cause a disease.

The cohort study design is the best available scientific method for measuring the effects of a suspected risk factor.

In a prospective cohort study, researchers raise a question and form a hypothesis about what might cause a disease.

Then they observe a group of people, known as the cohort, over a period of time. This may take several years. They collect data that may be relevant to the disease.

In this way, they aim to detect any changes in health linked to the possible risk factors they have identified.

For example, scientists may ask participants to record specific lifestyle details over the course of a study. Then, they can analyze any possible correlations between lifestyle factors and disease.

Comparing with other study types

Randomized controlled trials (RCT) are considered the best, most rigorous way of investigating interventional medicine, such as new drugs, but it is not possible to use them to test for the causes of disease.

Cohort studies are observational. The researchers observe what happens without intervening.

In experimental studies, such as RCTs, the scientists intervene, for example, by giving participants a new drug and assessing the outcomes.

When looking for the causes of disease, it would be unethical to deliberately expose participants to a suspected risk factor, as would be the case in an RCT. Instead a prospective cohort study is observational rather than interventional.

For drug testing, RCTs are the best option. Humans are used to test the safety and potential benefit of a treatment.

While the harms of a treatment sometimes outweigh the benefits, this form of testing is considered acceptable because the drug has already been tested many times and the researchers are quite sure that it is safe enough to try.

In addition, participants agree to join the trial, sometimes because they have a condition and there is a good chance the drug will improve their health.

Case-control studies are another type of observational study, also used to investigate the causes of disease.

Cohort studies are considered to be better than case-control studies because they are usually prospective. Case studies are limited because they are usually retrospective and involve a smaller number of people.

Examples

Some cohort studies have been very large and continued for a long time, producing a good deal of data that serves researchers in different fields.

Nurses Health Study

One famous example of a cohort study is the Nurses' Health Study, a large, long-running analysis of women's health, originally set up in 1976 to investigate the potential long term consequences of the use of oral contraceptives.

This study recruited its second generation cohort for the Nurses' Health Study II in 1989, and its third-generation cohort of nurses from across the United States and Canada in 2010.

The nurses in the first NHS were married women aged 30 to 55 years. The NHS II and III aimed to look at a more diverse cohort including women aged between 20 and 46 years.

Numerous and important insights into health and wellbeing have already been gained by researchers using data from the Nurses' Health Study, which is run by the Harvard School of Public Health, and the Brigham and Women's Hospital, both based in Boston, MA.

The following headlines are from news stories published recently by MNT. They report on some of the findings from this huge study of hundreds of thousands of women:

Because the Nurses' Health Study asks participants about their lifestyle choices, it has yielded much information about the harms and benefits of various factors, including specific types of food in the diet.

Cohort studies are also good at finding relationships between health and environmental factors such as chemicals in the air, water and food. These are issues that the World Health Organization (WHO) helps researchers to investigate with large-scale cohort studies.

Pooling data from different studies can increase the sample size, and this can make the results more reliable, especially for rare conditions such as some types of cancer.

Framingham Heart Study

Another example is the Framingham Heart Study, which recruited over 5,209 male and female participants in 1948 from around the area of Framingham, MA. It has continued to serve as a source of data for cardiovascular risk factors.

A second cohort was recruited in 1971 and a third in 2002. The study has made important contributions to the understanding of heart health. The researchers are now looking into how genetic factors may affect cardiovascular risk.

Big cohorts of babies

A birth cohort study is a long-term follow-up of people born in the same year. One has followed 17,000 people all born in the same week in 1958.

The latest, the Millennium Cohort Study, is following 19,000 millennium babies, children born in the UK between 2000 and 2001. In addition to data on the health of these children and their parents, the study is also looking into child behavior and cognitive development, as well as a range of social factors.

Limitations

Cohort studies are graded as the most robust form of medical research after experiments such as randomized controlled trials, but they are not always the best form of observational work.

Cohort studies do have some limitations:

They are less suited to finding clues about rare diseases. A case-control study identifies cases of disease first and then analyzes exposure to risk factors, whereas cohort studies follow exposure data and watch for any emerging cases of disease.

They are typically unsuitable for identifying the causes of a sudden outbreak of disease. A case-control study can give quicker results.

They are expensive to run and usually take many years, often decades, to produce results.

They can only offer clues about the causes of disease, rather than definitive proof of links between risk factors and health. This is true of any observational medical research.

Participants may leave the cohort, perhaps move away, lose touch, or die from a cause that is not being studied. This can bias the results.

If you would like to find out more about different types of medical research, MNT has produced a number of pages that answer your questions:

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